People are infected by foodborne trematodes after eating raw freshwater fish, crustaceans or vegetables containing the parasite larvae (metacercaria) that cause fever and abdominal pains. There are four main of species of these trematode worms (also known as flukes) and symptoms vary according to the type of species and the part of the internal organ that the flukes inhabit. Foodborne trematodiases have been reported in over 70 countries to date. The most severely affected areas, however, are in Southeast Asia and South America,
Causes of Infection
Trematode worms (flukes)
The first intermediate host is a freshwater snail, the second either a freshwater fish or a crustacean
People are infected by foodborne trematodes, after eating vegetables, raw fish, or crustaceans carrying a type of parasitic larvae (metacercaria), which have suckers on their mouths and abdomen. These trematodes have a digestive canal but no anus. Foodborne trematodiases are zoonoses, i.e., they are transmissible from vertebrate animals to people and vice versa. That said, only indirect transmission is possible. The most common species affecting humans among the various species of trematode worms are Clonorchis, Opisthorchis, Fasciola and Paragonimus.
In all cases, the first intermediate host is a freshwater snail, while the second host may differ: In clonorchiasis and opisthorchiasis, it is a freshwater fish; in paragonimiasis, it is a crustacean; and in fascioliasis, it does not require a second intermediate host but instead matures on the waterweed or other pasture to which they attach themselves.
A mammal is always the final host. People are infected when they ingest a second intermediate host that is infected with the larval forms of the parasite.
As foodborne trematodiases tend to be asymptomatic, early or light infections often go unnoticed. On the other hand, severe pain can occur, especially in the abdomen, if a large number of worms is present. Abdominal pain is most frequent in cases of infection by fascioliasis. Severe morbidity invariably accompanies chronic infections.
The symptoms are mainly specific to the affected organ and indicate where the adult worms are located in the body.
In clonorchiasis and opisthorchiasis, adult worms live in the smaller liver bile ducts, resulting in inflammation and fibrosis of nearby tissue. This can eventually result in bile duct cancer.
In fascioliasis, adult worms live in the larger bile ducts and the gall bladder causing inflammation, fibrosis, blockages, colic pain, and jaundice. Liver fibrosis and anemia are also common.
In paragonimiasis, lung tissue is the final location of the worms. This presents symptoms that can be mistaken for tuberculosis: chronic cough with blood in the sputum, chest pain, dyspnoea (shortness of breath), and fever. Migration of the worms is possible and cerebral locations are the most severe, causing serious symptoms that resemble brain tumors, including headaches, spasms, and paralysis.
Anthelminthic drugs can be administered. The drugs used vary with the type of worm:
A combination of praziquantel and triciabendazole
Food safety, including thoroughly cooking fish and crustaceans before eating them, is essential. Anthelmintics can be used preventively where there are a large number of patients. For clonorchiasis and opisthorchiasis, praziquantel is occasionally used, while for fascioliasis and paragonimiasis, triciabendazole is occasionally used as a preventive medicine for entire populations.
Regions at High Risk of Infection
Cases of foodborne trematodiases have been reported in over 70 countries, although Southeast Asia and South America are the most affected. Transmission is a reflection of behavioral and ecological patterns, including eating habits, food production and preparation methods and the distribution of intermediate hosts, leading to varying patterns even within each country.
Estimated Number of the People Infected
One or more foodborne trematodiases, affected at least 56 million people worldwide in 2005. In 2015, 600,000 people were treated for this disease.
Estimated Number of Deaths
More than 7,000 people died of this disease in 2005.
Initiatives by Pharmaceutical Companies and NGOs
WHO (World Health Organization) has been working to control foodborne trematodiases as a part of its integrated approach for the control of NTDs. These activities include:
- Developing strategic directions and recommendations for elimination
- Supporting mapping initiatives in endemic areas
- Supporting prompt intervention and pilot control programs in endemic areas
- Supporting the monitoring and evaluation of programs
- Documenting the burden of foodborne trematodiases and impact of intervention programs
WHO also promotes using preventive chemotherapy strategies to make certain that the worst consequences (cancers of the bile duct and other diseases) are completely prevented.
Novartis Pharma AG has donated triclabendazole for treating human fascioliasis and paragonimiasis to WHO. Since 2006, WHO has distributed more than 2.7 million triclabendazole tablets to targeted countries in need.
WHO- Neglected Tropical Diseases, accessed March 19, 2014,
CDC- Neglected Tropical Diseases, accessed March 19, 2014,